This application is the US national phase of PCT application PCT/EP2009/008213, filed 18 Nov. 2009, published 27 May 2010 as WO2010/057625, and claiming the priority of German patent application 102008057823.1 itself filed 18 Nov. 2008, whose entire disclosures are herewith incorporated by reference.
The invention relates to a capacitive sensor system, especially for detecting approaching objects and especially also for gesture detection. The invention relates to a sensor system in which on the base of electric near-fields the approach or movement of typically a hand or a finger is detected, and, from this, information is derived can be used for controlling switching actions or for recognizing a spatial gesture.
Especially for gesture detection there are optical processes in the visible or infrared range. Moreover capacitively acting systems are known that acquire the necessary information by the transmission or disturbance of an electric field. The is circuitry expenditure and the cost connected with the realization of such systems are hitherto high. Another problem in the conventional systems consists in the fact that in applications that require battery operation the electricity requirement of such sensors is crucial for practical applicability. Moreover in some applications component costs and space requirement are important, which in case of mass applications, for example in toy industry, can be determining the field of application.
For a capacitive sensor system the evaluation of the capacitance change of a RC low-pass filter structure is known. As excitation signal a sinusoidal voltage or a square signal is used. As signal indicator for the change, the amplitude or the phase or the time difference compared to a reference signal is evaluated. In both approaches (amplitude or phase) it is the relative change ΔC/C of the capacitance change compared to a basic capacitance C that matters, since by this the sensor sensitivity or the maximum detection range of the sensor is determined. A possibly small basic capacitance is to be aspired therefore for a maximum sensitivity.
The object of the present invention is to provide a capacitively acting sensor system that can be realized with small component expenditure and thus small cost and space requirement and which can be characterized moreover also by a low power consumption, in order to be able to work with batteries with small charging capacitance and/or long service life.
This object is attained solved according to the invention is by a circuit arrangement with the characteristics of claim 1.
With several such systems a two- or three-dimensional position detection is possible. It is also possible to provide several sensor electrodes and to connect them by a multiplexer device successively to the circuit. This multiplexer circuit can be triggered by the microcontroller.
Advantageous embodiments of the circuit arrangement according to the invention are object of the dependent claims.
Further particulars and characteristics of the invention result from the following description in association with the drawing. Therein:
In
First the operation of this arrangement represented in
The parasitic capacitors are on the one hand formed by the field coupling between a signal electrode indicated at Es and a ground electrode Eg of the circuit arrangement (C1) and on the other hand by coupling capacitors C2 and C3 of a hand approaching these electrodes.
The capacitors C4 and C5 are coupling capacitors of the hand or the circuit ground to earth. At first the charge and discharge in the basic state without approach (C2=C3=C4=0) is considered, with only C1 effective. The time for reaching a determined threshold value uS1 is assumed to be t1. The discharge occurs after half a cycle duration T of the square signal according to uE and after a time t2 again reaches a threshold value uS2.
Thus we have:
uS1=u0(1−e−t1/RC1)uS2=u0e−t2/RC1 (1)
From this results for the switching times
t1=−RC1 ln(1−uS1/u0)t2=−RC1 ln(uS2/u0) (2)
With a capacitance change by ΔC to C1+ΔC the time difference for the threshold values amounts to
Δt1=−RΔC ln(1−uS1/u0) and Δt2=−RΔC ln(uS2/u0) (3)
The total time difference is
Δt=Δt1+Δt2=RΔC[ln(u0/uS2)−ln(1−uS1/u0)] (4)
Equation 4 shows that as Δt becomes greater, the closer the threshold uS1 is to u0 and the smaller uS2 is compared to u0. This means that in choosing the threshold values a suitable hysteresis of a threshold value switch is favorable.
In case of uS1=uS2 it is advantageous either to put the threshold as near as possible to u0 or 0, as then one of the two terms in equation 4 as big as possible. Moreover the time difference and thus the sensitivity of the sensor is higher, as the charging resistance and the capacitance change ΔC increase. The charging resistance is maximized each time during a semi-period T/2 of the square signal still as an almost complete charge and discharge at the gate terminal of a field-effect transistor occurs.
Since R is to become as big as possible, this requirement depends decisively on the capacitance C1 effective between gate terminal and ground (see
A further contribution to C1 is supplied by the coupling capacity between the signal electrode ES and the electrode EG connected to ground. In order to minimize this, a so-called shield electrode can be connected between, that is connected to the output of the source follower and therefore has almost the same potential as the gate terminal, by which the coupling ES and EG is considerably reduced. This is another advantageous aspect of the FET stage. The drain terminal of the FET can also be used, in case of a more distanced connection of the signal electrode ES, to drive the braid of a coaxial cable and to reduce the cable capacitance in this way, which would also deliver a contribution to C1.
All this shows that the use of a FET as a source follower as an input stage entails considerable benefits and moreover reduces component expenditure to a minimum, so that both electricity requirement and costs remain very low.
For the evaluation of the time shift of the charge and discharge process in case of approach, a XOR gate terminal is used whose inputs are switched by integrated Schmitt triggers, so that no additional comparator for the switching thresholds uS1 and uS2 is necessary, and thus further components can be saved. The time difference is represented by a DC voltage obtained by a lowpass filter connected to the XOR output. With a supply voltage uB is then analogously to equation 4
u=RC1uB/T[ln(u0/uS2)−ln(1−uS1/u0)] (5)
Putting here 1/T=f, it can be seen that the DC voltage formed at the low pass-output is proportional to the frequency f of the square signal delivered by the μC. As due to inevitable tolerances, for example in the threshold values, this voltage can vary in a production process, a possibility for tolerance compensation consists in varying the signal frequency by the μC in such a way that in case of no approach always a constant output voltage results.
Crucial for the sensitivity of an approaching detection is the capacitive change ΔC at the gate terminal, which according to equation 4 leads to a corresponding time difference Δt and thus, at the low-pass output, to a proportional voltage change Δu˜ΔC. As already stated above, this change depends above all on the coupling capacitances C2 to C5 effective in case of an approach. An equivalent circuit (
The coupling to the ground electrode EG does not necessarily have to take place by a separate electrode, but may occur, depending on the application, also by a different coupling by for example the batteries. The arrangement according to the invention of
In the following two examples for the application of the sensor arrangement are given.
In the first example according to
Another example for the application of the proximity sensor is the detection of gestures by a four-electrode system, the principle of which is shown in detail in
For this purpose, in the system according to
r12=(x−a)2+y2+z2 (6)
r22=(x+a)2+y2+z2 (7)
r32=x2+(y+a)2+z2 (8)
r42=x2+(y−a)2+z2 (9)
By taking the difference of each time equation 6 and 7 or 8 and 9 one immediately obtains the x/y coordinates for
x=(r12−r22)/4a
y=(r32−r42)/4a (10)
As the equations 6 to 10 show, the x/y coordinates can be calculated in a simple way independently of z. For this purpose the distances r1 to r4 must be determined from the signals that are applied to the output of the four sensors S1 to S4. Only those signal differences are considered that result in case of approach compared to the basic state. These differential signals are designated with e1 to e4 and are deduced from the capacitance variations described above at the respective gate terminals of the field-effect transistors. Decisive for this purpose is each time the coupling capacitance of the finger to the electrodes, which becomes smaller with growing distance from the electrode. As the delivered amplitude of the signal difference according to the above statements is proportional to the capacitance change, these values decrease with growing distance. For this by approximation a power is law is assumed according to
e(r)=e0(r0/r)α (11)
With an exponent α that amounts in practice depending on the electrode arrangement to 2 . . . 3.
The resolution of equation 11 to r gives
r=r0(e0/e)1/α (12)
With the equations 10 now the coordinates can be calculated from the signals e1 to e4:
x=[(e0/e1)2/α−(e0/e2)2/α]r02/4a (13)
y=[(e0/e3)2/α−(e0/e4)2/α]r02/4a (14)
The constants e0, r0 and a here depend on the electrode shapes and orientations of the electrodes relative to each other.
In
Benefits of the Arrangement
In short with the arrangement according to the invention of a capacitive proximity sensor the following benefits are emphasized once more:
1. The expenditure in components is, with only one FET is input stage, one XOR gate terminal and few resistors and a capacitor, extremely small. The microcontroller necessary for signal generation and processing is, in case of integration of the sensor in other systems, often already present, and can also be used for the simple necessary sensor functions.
2. The FET switched as a source follower delivers, due to the counter-coupling, not only a very small inherent capacitance of the sensor, but moreover can serve as an output for a shielding operation in order to reduce the basic capacitance determining the sensitivity of the sensor. Moreover this measure offers a high temperature stability of the sensor function and reduces sample dispersion.
3. A consequence of the small number of active components is a very low power consumption, which because of very short transient processes of the arrangement, by a pulse-operated operation can be reduced to few μA, which entails considerable benefits when powered by a battery.
4. By appropriate choice of the electrode arrangement the approaching function of a necessary coupling to earth can be solved. This is essential for applications with battery power.
5. A self-calibration of the sensor necessary due to tolerances can take place in a simple way by frequency adjustment.
6. By varying the charging resistance at the gate terminal, a very flexible adjustment to different electrode capacitors due to different electrode sizes can take place. Moreover in this way also an optimal frequency adjustment with respect to foreign disturbers can be done.
7. The reaction time of the sensor can be reduced to a few milliseconds by flexible choice of a possibly high signal frequency.
8. In case of multielectrode arrangements like in sensors for gesture detection the total sensor can be housed in a compact way on only few cm2.
A special measure in case of the circuit according to the invention is the particularly simple and thus power and cost-saving realization of the proximity sensor with only one FET stage and a downstream EXOR gate without an additional comparator being required. On the other hand the FET stage delivers, if it is switched as a source follower, an extremely small input capacitance and thus allows a high series resistance determining sensitivity that is higher than that of conventional sensors by as much as a factor of 50. Moreover the stage in this configuration offers at the same time also a shielding function that in critical installation circumstances can hold the basic input capacitance low, and thus no relevant sensitivity loss occurs.
The comparison with conventional sensors shows that the realization of the proximity sensor of the invention according to the RC process also with a smaller number of components even leads to a higher operating efficiency.
The concept according to the invention is characterized by a particularly low power consumption and it is suitable especially for battery applications. The cost, determined mainly by the number of the active components of a circuit arrangement and the space requirement, is clearly lower than that of conventional concepts. The circuit concept according to the invention is suitable in a particularly advantageous way for systems with a simultaneous operation of several sensors, as for gesture applications.
Number | Date | Country | Kind |
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10 2008 057 823 | Nov 2008 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2009/008213 | 11/18/2009 | WO | 00 | 2/25/2011 |
Publishing Document | Publishing Date | Country | Kind |
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WO2010/057625 | 5/27/2010 | WO | A |
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